This invention relates generally to sealing devices disposed in association with two relatively rotatable members and providing a fluid seal therebetween. More particularly, the present invention relates to an apparatus for achieving sealing between a rotating member and a housing circumscribing the rotating member.
Gas turbine engines employ sealing devices in various capacities where it is necessary to restrict the flow of fluid or gases from one portion of the engine to another. A common use is for separating the primary engine flow path from the secondary flow path. The primary engine flow path directs the flow of gases to the compressor and turbine stages from which the engine derives thrust or power. The secondary flow path comprises a series of conduits for delivering compressed air throughout the engine for performing a variety of functions. Compressed air is used, for example, to cool individual components, provide a bleed air source, buffer the lubricated bearing cavities, control the ventilation among engine cavities and structures, and affect the thrust balance of the engine. Loss of compressed air from the secondary flow path through leakage can have a substantial adverse effect on the performance of these functions. In a turbine engine, at least one sealing device typically is required for each turbine and compressor stage of the engine.
Another common use for sealing devices in turbine engines is for separating the secondary flow path from engine cavities containing fluids such as lubricating oil. In pressurized aircraft, bleed air taken from the secondary flow path supplies the aircraft environmental control system. Even small amounts of oil in the bleed air can render it unsuitable for this purpose. Further, oil leakage can lead to coking of the seal, and ultimately reduced seal life. To prevent this, buffered sealing devices typically are incorporated adjacent lubricated bearings and engine oil sumps.
The most common type of seal used for these purposes is the labyrinth seal. A labyrinth seal is comprised of a multiplicity of radially extending annular knives mounted on a rotating shaft, and an annular seal land closely circumscribing the knife edges. The gap between the knife edges and the lands restricts the flow of secondary flow path air therethrough, creating a seal.
A problem inherent with labyrinth seals is that these gaps must be large enough to accommodate radial excursions of the shafting on which the seal is mounted. Radial excursions can be substantial, particularly in propulsion gas turbine engines. Large radial excursions may be caused by critical speed response, aircraft maneuver induced loading, impact loads, engine vibration, and thermal and speed transients. To minimize leakage, a relatively thick layer of material is added to the seal land, into which the seal knives cut grooves during these radial excursions. The added layer of material typically consists of either a coating of silver or ceramic abradable material, or a welded on honeycomb type material.
Labyrinth seals are also very costly to manufacture. The rotating portions are machined from expensive high-strength forging to a complex shape having exacting dimensional requirements. Additionally, in many of these seals a costly manufacturing technique is required for welding on the leakage reducing honeycomb material to the seal land. Nevertheless, gaps remain fairly large in operation and the leakage rates higher than desired for many applications.
An improved sealing concept less commonly seen in turbine engines is the brush seal. Brush seals may take a variety of forms for use in a variety of types of applications. When configured for use in a turbine engine, brush seals are typically comprised of a plurality of generally radially oriented metal wires tightly packed and bound at their outer ends into an annular retainer. This brush structure, which comprises the nonrotating part of the seal, is mounted to a wall or plenum structure which houses a rotating shaft. The radially innermost tips of the wires making up the brush form a bore for receiving the rotating shaft in a slightly radially interfering relationship. Because of the flexibility of this brush portion, the seal can accommodate radial excursions of the shafting without the need for a radial gap between the seal and shaft. Thus, leakage is confined predominately to migration of fluid through the brush portion itself, and is controlled by ensuring that the wires are densely packed. Brush seals are typically selected based on their sealing capability, providing improved sealing over even multi-stage labyrinth seals.
Brush seals, however, suffer from a high rate of wear and must be replaced often. The ends of the metal wires begin wearing immediately upon use, causing leakage between the brush and the shaft to increase over time. In turbine engine applications wear induced leakage may ultimately reach an unacceptable level, necessitating replacement of the seal. The continuous rubbing contact also tends to abrade the surface of the shaft, or rotating component in contact with the brush, potentially resulting in expensive replacement or rework of the rotating parts as well. Yet another problem associated with brush seals is that they have a tendency to occasionally lose bristles. This tendency may preclude the use of a brush seal in applications where the resulting risk of damage to neighboring components is high.
On the other hand, as the demand for small size, increased power output, and improved specific fuel consumption increases in the turbine engine arts, the brush seal becomes increasingly attractive. Such is the case because of the brush seal""s improved sealing effectiveness, smaller size, and it is hoped, decreased engine cost when compared to engine designs using conventional labyrinth seals.
Among the sealing devices more recently developed is the finger seal. Finger seals are comprised generally of a plurality of flexible members fixed at one end, the opposite ends sealingly engaging a surface that is rotatable relative thereto. The fingers of a finger seal are an integral part of the seal, usually formed by machining a series of curved slots in a forged ring or a length of sheet stock, the slots being of consistent length and extending from a common edge of the material. A complete seal is usually made up of two or more layers of fingers relatively positioned such that the gaps between fingers in one layer are blocked by the fingers of the next layer.
During engine operation, the fingers deform in a generally radially outward direction due to various factors including centrifugal growth of the rotating surface. In order to maintain an effective seal, it is also necessary for the fingers to restore themselves radially inward as the rotating surface shrinks. Fluid pressure acting on the radially deformed fingers, however, causes the fingers to deform axially often resulting in forceful contact between the fingers and adjacent structures. This axial deformation creates a frictional force that hampers radially inward recovery of the fingers as rotational velocity of the rotatable surface subsides. Consequently, the fingers are unable to re-engage the movable surface as it shrinks. Accordingly, the desired seal therebetween is undermined leading to engine inefficiencies.
A solution to this problem can be found in copending U.S. patent application Ser. No. 091248,441 filed Feb. 11, 1999 and assigned to the assignee of this patent application. The xe2x80x2441 application discloses a sealing apparatus having a plurality passages between the high pressure side of the seal and a cavity on the low pressure side but isolated from the low pressure air. The pressure in the cavity is at an intermediate pressure slightly less than the high pressure. As a consequence the total pressure drop across the seal is reduced reducing the axial deformation.
Though the xe2x80x2441 application solved the problem of axial deformations, some fingers seals such as the ones disclosed in Arora, U.S. Pat. No. 5,755,445 have non-contacting foot portions that are designed to lift off from the shaft as speed of the shaft increases. Testing of these seals revealed that the foot portions were not able to lift off. This was attributed to the hydrodynamic force acting to lift the foot portions away from the shaft not being high enough to overcome the pressure induced force in the radial direction across the foot portions.
Accordingly, a need exists for a foot portion in a finger seal configured to overcome the deficiencies of the prior art foot portions.
An object of the present invention is to provide a finger seal with a foot portion configured to overcome the deficiencies of the prior art foot portions.
The present invention meets this objective by providing a sealing apparatus having fore and aft annular cover plates. Disposed between these plates are a fore spacer, a sealing element, and an aft spacer. The sealing element comprises two comb-like diaphragm members extending radially inward from a continuous, circumferential band portion. Each diaphragm member has a plurality of uniformly spaced integral finger members, the foot portions of which sealingly contact the rotating member. The finger members of each diaphragm member have gaps therebetween. The diaphragm members are positioned so that the finger members of one blocks the finger member gaps of the other and vice versa. The band portions have a plurality of circumferentially disposed holes which define an axial passage when the diaphragm members are assembled. The fore spacer has a plurality of radial passages which deliver high pressure fluid to the axial passage in the diaphragm members. From the axial passages the high pressure fluid flows through radial passages in the aft spacer to cavities in the aft spacer. Thus, the net axial force or thrust exerted on the sealing element is greatly reduced when compared to the prior art configurations that do not have these pressure balancing passages. With this reduced axial force, the frictional force between aft spacer and the sealing element is reduced which eliminates or greatly reduces binding of sealing element against the aft spacer. The sealing element is now free to expand and contract radially as required. Further, each of the foot portions has a balancing groove so that at higher rotational speeds of the rotating member the pressure and hydrodynamic forces on the inner surface of the foot portion causes it to lift away from the rotating member and ride on a thin film of air thus effectively sealing the rotating member.